1. Field of the Invention
The present invention relates to a process for the preparation of an aqueous polymer dispersion, in which an aqueous dispersion of a polymer which contains at least two monomers A and B which differ from one another, have at least one ethylenically unsaturated group and are incorporated in chemically bonded form is produced, in a manner known per se, so that the total content of monomers which are free, ie. not chemically bonded, and have at least one ethylenically unsaturated double bond (referred to in this publication as content of residual monomers or residual monomer content) in the aqueous polymer dispersion is from &gt;0 to .ltoreq.1% by weight, based on the aqueous polymer dispersion, and this residual monomer content is then reduced by the action of a free radical redox initiator system comprising at least one oxidizing agent and at least one reducing agent.
The present invention furthermore relates to aqueous polymer dispersions resulting when such a procedure is used.
2. Discussion of the Background
Aqueous polymer dispersions are systems which contain polymer particles in disperse distribution as the disperse phase in an aqueous dispersing medium.
As in the case of polymer solutions when the solvent is evaporated, aqueous polymer dispersions have the property of forming polymer films when the aqueous dispersing medium is evaporated, and aqueous polymer dispersions are therefore widely used as binders, for example for surface coatings or for materials for coating leather.
In principle, a person skilled in the art classifies aqueous polymer dispersions as aqueous secondary dispersions and aqueous primary dispersions. The aqueous secondary dispersions are those in the preparation of which the polymer is produced outside the dispersing medium, for example is present in solution in a suitable nonaqueous solvent. This solution is then transferred to the aqueous dispersing medium, and the solvent is separated off, as a rule by distillation, while dispersing is effected. In contrast, aqueous primary dispersions are those in which the polymer is produced in the aqueous dispersing medium itself, directly as the disperse phase.
The common feature of all preparation processes is essentially that monomers which have at least one ethylenically unsaturated group are used for the synthesis of the polymer, or that this synthesis is effected exclusively from such monomers. The incorporation of such monomers having at least one ethylenically unsaturated group is usually carried out by an initiated polymerization reaction, the type of initiation used being determined in particular by the desired performance characteristics of the end product and therefore being adapted to these. For example, ionic or free radical initiation is suitable. However, the incorporation may also be effected by catalytically initiated polymeranalogous reaction. Free radical initiation is particularly frequently used, and the incorporation of the monomers having at least one ethylenically unsaturated group is therefore carried out as a rule by the free radical aqueous emulsion or suspension polymerization method in the case of aqueous primary dispersions and as a rule by the free radical solution polymerization method in the case of aqueous secondary dispersions.
As it is generally uneconomical to attempt to achieve complete conversion while maintaining the actual polymerization reaction conditions which determine the desired properties of the end product, in relation to the incorporation of the monomers having at least one ethylenically unsaturated group, the aqueous polymer dispersions resulting after the end of this main polymerization reaction usually contain free monomers which are not incorporated and have at least one ethylenically unsaturated double bond (this is the case in particular when monomers such as acrylic acid, methacrylic acid or the amides thereof are involved, said monomers having high water solubility). Owing to the high reactivity of the ethylenically unsaturated double bond, such residual monomers are not toxicologically safe and are therefore undesired both by the producer and by the user.
There are in principle already various methods for reducing the residual monomer content of aqueous polymer dispersions.
EP-A 584 458 discloses, for example, the reduction of the residual monomer content of aqueous polymer dispersions by stripping by means of steam.
DE-A 38 34 734 recommends reducing the content of residual monomers in aqueous polymer dispersions by means of the action, after the end of the main polymerization reaction, of a free radical redox initiator system which is less suitable for initiation of main polymerization reactions, for example from the point of view of the usually desired performance characteristics and economical polymerization conditions, hydroxymethanesulfinic acid or the salts thereof being recommended in particular as the reducing agent to be used.
By means of these methods recommended in the prior art for reducing the residual monomer content of aqueous polymer dispersions, residual monomer contents of 1% by weight, based on the aqueous polymer dispersion, can usually be achieved regularly and without great difficulties. By means of these methods, it is of course also possible to achieve residual monomer contents of less than 1% by weight, based on the aqueous polymer emulsion.
Below the 1% by weight limit, however, increasing difficulties are encountered in reducing the residual monomer content when the residual monomers are composed of at least two monomers A and B which differ from one another, since this difference in the residual monomers generally also results in different solubilities thereof both in the aqueous dispersing medium and in the dispersed polymer particles. For different residual monomers A and B, this usually leads to different distribution equilibria between the two phases of an aqueous polymer dispersion, with the consequence that, when the known processes are used for reducing the residual monomer content, the reduction in the contents of the different residual monomers does not take place uniformly, this effect becoming increasingly significant below the abovementioned 1% by weight limit. This means that the method used for the reduction either preferably affects residual monomers which are preferably present in the aqueous dispersing medium or preferably affects those residual monomers which are preferably present in the dispersed polymer particles.